In real life we gain three-dimensional information by a variety of cues. Two of the most important ones are binocular parallax and motion parallax. The binocular parallax is caused bye the distance of the two eyes. The two images caught bye the left and right eye are slightly different. The brain combines the two different images and gets a stereo impression

In stereoscopy, binocular parallax is established by artificial methods, and different images with parallax are sent to the left and right eye individually. Our brains obtain these images and get the 3D impression via treating the difference as spatial location.